75 engine with early 78 carb compatability

[Shovel Jockey]

I had to pull the engine from my stock early 78E for extensive repairs. I had a spare engine from a stock 75B and installed it. I am still using the 78 carbs and exhaust on the 75 engine. It seems to be running fine but I realize these are two different engines,(valve settings; compression ratios, plugs). Is there a possible problem running the 78 carbs on the 75 engine?

 

[gggGary]

Short answer no, except the 78 carbs are better in several areas and the motor may just take a serious liking to breathing through those 78 carbs. IMHO there is VERY little difference in the 75-78 447 motors.

 

[Shovel Jockey]

I think you may be right! The 75 engine is running stronger and smoother especially in the bottom and mid range than the 78 engine ever did. I guess I was just concerned about it running too lean. Checking out the carb specs I realized the motors were similar but the carbs were noticeably different on jetting and adjustment.

 

[gggGary]

PS Some 75 dip sticks came with wrong markings, compare to your 78 dipstick use it's markings if different.

 

[5twins]

The only problem I could foresee is a clearance issue between that front lower angle bracket linking the '78 carbs and the cam chain tensioner cap nut on the '75 motor. If this is a problem, you can swap the entire tensioner assembly from the '78 motor on to the '75 motor.

 

[Shovel Jockey]

You are exactly right on the clearance issue 5twins. I swapped tensioners with an '80 model engine so it would have the jam nut.